Electromechanical systems, such as electrically operated hydraulic valves for example, are subject to sticking when valves are left in the same position for a period of time. Consequently, when electricity is applied to the valve solenoid, to make it move, the valve may need to overcome a certain amount of friction from the sticking before it actually moves. As a result, the mechanical motion of the valve does not linearly track the applied current and instead follows a hysteresis curve. This can result in adverse operating condition in precision systems, such as vehicle transmissions for example. To combat this problem, the electromechanical system must be operated with a range of parameters dictated by the design of the components. One of these parameters is the frequency of the applied signals for control of the device. The frequency components of the electrical signals can be used to keep the electromechanical system in constant small-scale motion such that hysteresis is greatly reduced. This excitation component of the signal is known as “dither”. In this way, the controlled current to the electrical load ensures the proper operation of the electromechanical system.
For electrical loads such as an inductance coil of an electromechanical system, such as a solenoid relay or valve actuator, many prior art circuits have controlled average current through the load inductance by controlling an amplitude of the drive current between two setpoints by use of a driver device connected in series with the load inductance. Typically, the current through the load inductance is sensed and the driver device is controlled to increase the load current when it is below a certain level and decrease the load current when it exceeds a certain level. In this manner, the solenoid current will oscillate repetitively between maximum and minimum levels (i.e. hysteresis) and thereby a desired average current level is achieved.
When the position of the mechanical system is to be switched, the setpoints are changed for the drive current to provide the transition. Due to the mass of the mechanical components and the electrical response of the electrical system, the transitional response of the electromechanical system is limited by a relatively constant slew rate. Moreover, the above current control scheme, based on electrical hysteresis control, only controls the maximum and minimum of the current waveform. Due to different electrical characteristics, the average or RMS current value of the waveform can shift significantly depending on the load. This can result in improper operation of the electromechanical system. Further, the above current control scheme does not provide a fixed frequency of operation.
One frequency problem with the prior art is that changing the amplitude of the setpoints will change the frequency of operation of the system due to the relatively constant slew rate. This is not a problem with larger valves, as the mechanical resonance of the system is much lower in frequency than the electrical response. However, newer systems have been requiring smaller and lighter valving, wherein the mechanical and/or hydraulic frequency response of the system approaches the electrical frequency response of the system. As a result, the dither frequency, and moreover the variable nature of the dither frequency, used to prevent sticking of the valve may actually feedback into the resonant mechanical and hydraulic systems, causing unpredictable excitation of the electromechanical system and systems coupled thereto.
In addition, the switching frequency is affected by the power supply (battery) level, wherein the switching frequency can change radically between low and high battery conditions. In this case, switching frequency can interfere with dither frequency. However, just providing a fixed frequency control would also be insufficient as the transient response of the system is still inadequate. Therefore, it would be desirable if the frequency of operation could be adapted easily as needed across the operating range of the electromechanical system.
What is needed is a frequency-controlled load driver current for an electromechanical system. It would also be of benefit to incorporate a fast transient response scheme for current control. It would also be advantageous to allow a simple change in the frequency of operation and to provide two modes of operation: one for steady state conditions and one for transient conditions.